1. Field
Embodiments of the present invention relate to an electronic brake system and a control method thereof, and more particularly to, an electronic brake system and a control method thereof capable of adjusting pressure of a master cylinder using an electrically-driven booster.
2. Description of the Related Art
Typically, in an electronic brake system including an electrically-driven booster, when a driver presses on a brake pedal, a pedal force sensor detects pedal force of the driver and an electronic control unit (ECU) drives a motor of the electrically-driven booster based on the detection result of the pedal force sensor to deliver multiplied force to a master cylinder. The master cylinder transfers hydraulic pressure to wheel brakes based on the multiplied force delivered from the electrically-driven booster.
Then, an electronic stability controller (ESC) adjusts the hydraulic force transferred to the front and rear wheel brakes according to a braking mode to achieve suitable braking operation. In other words, the ESC allows brake oil to be supplied to the respective wheel brakes in a normal operation mode. However, in an anti-lock brake system (ABS) mode, i.e., when at least one of the wheels undergoes a locking phenomenon, the ESC allows ABS braking operation to be performed by closing some brake oil passages.
In such an electronic brake system including the electrically-driven booster, when a driver applies pressure to a brake pedal by pressing on the same, a motor provided in the electrically-driven booster rotates a ball screw in one direction to move the ball screw forward. The ball screw is configured to move forward and backward through conversion of rotational movement of the motor into linear movement. The ball screw causes an output rod connected to a piston of a master cylinder to be moved forward, thereby moving the piston forward. As the piston is moved forward, internal pressure of the master cylinder is increased, and the increased pressure is transferred to wheel brakes.
When the pressure applied to the brake pedal is released, the motor rotates the ball screw in a reverse direction to move the ball screw backward. Accordingly, the piston of the master cylinder is moved backward, and internal pressure of the master cylinder is decreased.
However, in an ABS mode, the brake oil in the master cylinder may fluctuate and thus internal pressure of the master cylinder may become irregular because of the brake oil returned from the wheel cylinder to the master cylinder. In such a case, if the pressure of the master cylinder is greater than reference pressure, the ball screw of the electrically-driven booster is moved backward to decrease the pressure of the master cylinder. If the pressure of the master cylinder is less than the reference pressure, the ball screw of the electrically-driven booster is moved forward to increase the pressure of the master cylinder.
In general, an isolation gap is formed between an output rod connected to the piston of the master cylinder and an input rod connected to the brake pedal. In abnormal operation of the electronic brake system, the input rod moves a distance corresponding to the isolation gap between the input rod and the output rod, and then physically contacts the output rod, thereby directly transferring pedal force to the master cylinder.
However, when the pressure of the master cylinder is greater than the reference pressure in an ABS mode, and if the ball screw of the electrically-driven booster is moved backward to decrease the pressure of the master cylinder, the output rod contacting the piston of the master cylinder is also moved backward together as the ball screw is moved backward.
At this time, because the driver keeps pressing on the brake pedal and thus the input rod connected to the brake pedal is in a forward-shifted position, the output rod may move a distance beyond the isolation gap and may collide with the input rod, which results in a kick-back phenomenon such that a physical shock is transferred to the brake pedal connected to the input rod due to collision with the output rod. Such a kick-back phenomenon may cause a driver to experience a pulsation in the brake pedal, thereby giving the driver an unpleasant feeling.